1. Field of the Invention
The present invention relates to an exhaust gas-purifying catalyst.
2. Description of the Related Art
As an exhaust gas-purifying catalyst that treats exhaust gas of an automobile, a three-way catalyst with precious metal such as platinum supported by an inorganic oxide such as ceria or alumina has been widely used. In the three-way catalyst, the precious metal plays the role in promoting the reduction of nitrogen oxides and the oxidations of carbon monoxide and hydrocarbons. Further, the inorganic oxide plays the roles in increasing the specific surface area of the precious metal and suppressing the sintering of the precious metal by dissipating heat generated by the reactions. In particular, ceria has an oxygen storage capacity and is capable of optimizing the oxidation and reduction reactions.
In recent years, occasions when the automotive vehicle such as automobile is driven at high-speed increase as the performance of an engine increases. Additionally, in order to prevent pollution of the air, the regulations on the exhaust gas are made more stringent. Against these backdrops, temperature of the exhaust gas emitted by the automotive vehicle is on the trend of rising.
Further, the automotive vehicle is required to decrease the carbon dioxide emission in order to suppress the global warming. For these reasons, occasions when the supply of fuel to the engine is cut off in the state that the exhaust gas-purifying catalyst is heated to high temperatures are increasing.
That is, the exhaust gas-purifying catalyst is used at temperatures higher than in the past, and occasions when exposed to an atmosphere excessive in oxygen at high temperatures are increasing. For that, in order to provide the exhaust gas-purifying catalyst that delivers a sufficient performance even when used under such a condition, research and development are actively carried out.
For example, JP-A 5-168926 (KOKAI), JP-A 6-75675 (KOUHYO), and JP-A 2000-169148 (KOKAI) describe improving the heat stability of ceria to suppress the reduction in its oxygen storage capacity and the like. Specifically, JP-A 5-168926 (KOKAI) describes an exhaust gas-purifying catalyst containing an element of platinum group, activated alumina, cerium oxide, barium compound and zirconium compound. JP-A 6-75675 (KOUHYO) describes an exhaust gas-purifying catalyst in which a catalyst-supporting layer contains cerium oxide, zirconium oxide and catalytic metal, and at least parts of cerium oxide and zirconium oxide are present as a composite oxide or a solid solution. JP-A 2000-169148 (KOKAI) describes a cerium-based composite oxide represented as the general formula: Ce1−(a+b)ZraYbO2-b/2.
Further, JP-A 10-358 (KOKAI) and JP-A 2001-129399 (KOKAI) describe making platinum present as platinum composite oxide to suppress the sintering of platinum. Specifically, JP-A 10-358 (KOKAI) describes an exhaust gas-purifying catalyst using a high heat-resistant composite oxide that contains platinum and one or more element selected from alkaline-earth metal elements and group IIIA elements. JP-A 2001-129399 (KOKAI) describes an exhaust gas-purifying catalyst that includes a platinum composite oxide layer containing platinum and alkaline-earth metal element on an inorganic oxide support, in which a layer of oxide of metal X, which is at least one element selected from Mg, Ca, Sr, Ba, La and Ce, is interposed therebetween.
However, even if the heat-stability of ceria were improved, the sintering of platinum would occur when the exhaust gas-purifying catalysts are exposed to an atmosphere excessive in oxygen at high temperatures, for example at temperatures from 1,000° C. to 1,200° C., and a sufficient activity would not be achieved. Also, in order to produce platinum composite oxide with a high heat-stability, firing at high temperature is necessary. For this reason, a large majority of exhaust gas-purifying catalysts using platinum composite oxide are small in specific surface area and insufficient in activity.
To solve this challenge, the present inventors have proposed a composite oxide catalyst containing rare-earth element, alkaline-earth element and a precious metal, in which a part of the rare-earth element and a part of the alkaline-earth element form a composite oxide, and this composite oxide and a part of the precious metal form a solid solution. The composite oxide catalyst exhibits an excellent activity even in the case where used at high temperatures in an atmosphere whose oxygen concentration is high. However, in some cases, the activity of the composite oxide catalyst may be lowered when used with a refractory support made of alumina.